Liquid-crystal composition, methods of adjusting the resistance, and substituted phenols

ABSTRACT

The present invention relates to liquid-crystal mixtures having specific resistance values, comprising acidic compounds. The present invention furthermore relates to liquid-crystal displays containing these liquid-crystal mixtures. The invention furthermore relates to compounds of the formula I,  
                 
 
     in which the parameters are as defined in the text, to their preparation, and to their use for achieving certain specific resistance values in liquid-crystal mixtures. The application also relates to a method of adjusting the specific resistance of liquid-crystal mixtures using acidic compounds.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to liquid-crystal mixtures having certainresistance values, and to a method of adjusting the specific resistanceof liquid-crystal mixtures to prespecified values, typically in therange from 10⁹ Ω·cm to a few 10¹² Ω·cm. This also applies in particularto liquid-crystal mixtures which comprise or even consist of terminallyfluorinated compounds or compounds carrying terminally fluorinatedsubstituents, and in general to high-resistance liquid-crystal mixturesof low polarity.

[0003] In particular, the resistance is adjusted using acidic compounds,particularly preferably phenols. The present invention also relates tonovel substituted phenols. The: present invention furthermore relates toliquid-crystal displays containing liquid-crystal mixtures according tothe invention.

[0004] 2. Description of the Prior Art

[0005] Liquid-crystal mixtures having high specific resistance valuescause problems in some types of liquid-crystal display.

[0006] A major problem of these high-resistance liquid-crystal mixturesis the occurrence of electrostatic charges. This sometimes happenssimply when protective films, for example of polarizers, or compensationfilms are peeled off during production of the displays. However,electrostatic charging can also occur during operation of such displays,for example due to contact and/or friction with plastic parts or cloths.In automobile radio displays, for example, electrostatic charges of thistype can even occur due to finger contact with the display. In thesimplest case, this electrostatic charge can result in undesiredswitching on of a switched-off display or part of the display.

[0007] Frequently, however, irreversible changes to the display areobserved. These are caused, for example, by a change to the alignmentlayers by the electrostatic charges. This phenomenon is observed inparticular in TN and STN displays. In displays of this type, a highspecific resistance of the liquid-crystal mixture frequently alsoresults in information which has been displayed for an extended periodremaining even after the display has been switched off. Owing to thehigh specific resistance of the liquid-crystal mixtures, accumulatedcharge carriers can only be dispersed with difficulty, causing theoccurrence of so-called “afterimages” or “ghost images”, often alsoreferred to as the “sticking effect” or “image sticking effect”. Indisplays with active matrix addressing (for example TN-AMDs orIPS-AMDs), the active matrix's non-linear switches (for example TFTs),in particular, can be damaged or even destroyed. “Image stickingeffects” can also occur in AMDs.

[0008] A number of proposals for solving this problem have already beenmade in the prior art. Besides equipment measures, intended to preventthe occurrence of electrostatic charging in, for example, the productionof liquid-crystal displays, a number of suggestions have been describedfor optimizing the liquid-crystal mixtures.

[0009] Most of these suggestions propose the use of various dopants inorder to achieve the desired resistance values. However, the solubilityof the dopants in the liquid-crystal mixtures is usually problematichere. Furthermore, undesired effects, such as, for example, the loweringof the clearing point and changes in the other physical properties,frequently occur. The reproducibility of the setting of the desiredresistance is frequently also not good or the range of achievableresistance values is relatively narrow.

[0010] A further essential property of the compounds employed to adjustthe conductivity or specific resistance of liquid-crystal mixtures istheir vapor pressure. This must not be too high, as must that of theother constituents of the liquid-crystal mixtures, since otherwise achange in the composition can occur, resulting here precisely in anundesired change in the resistance. This is of particular importance inthe extremely widespread use of vacuum filling units in displaymanufacture. The changes which occur seem to be dependent on theduration and magnitude of the pressures which occur.

[0011] A typical example of compounds recently used to set certainspecific resistance values in liquid-crystal mixtures are crown ethers,used as described in WO 97-03 164. However, even when used in smallamounts, these result in very considerable reductions in the resistance.

[0012] In addition, owing to the interaction of the crown ethers withvarious impurities, both in the liquid-crystal mixtures and on theinternal display surfaces, i.e. essentially the alignment layers, theresults achieved are very highly dependent on the material used, and thereproducibility is frequently inadequate. There was thus a demand forsubstances for the reproducible adjustment of the specific resistancewhich are readily soluble in liquid-crystal mixtures and are compatiblewith many alignment layers.

[0013] Compounds of the formulae

[0014] are known, see, for example, Chemical Abstracts CAS-2894-87-3 andCAS-2200-70-6.

[0015] JP-A 08-067 577 proposes tri(polyoxyalkylene)amine for reducingthe specific resistance of liquid-crystal mixtures.

[0016] JP-A 08-337 778 describes liquid-crystal mixtures containingperoxide-destroying compounds for use in STN displays.

DESCRIPTION OF THE PRESENT INVENTION

[0017] The present invention had the object of providing liquid-crystalmixtures having prespecified specific resistance values. A furtherobject was thus to select or provide substances which allow the specificresistance of liquid crystals to be adjusted reproducibly.

[0018] Surprisingly, this has been achieved by using acidic compoundsand in particular acidic phenols, very particularly those of the generalformula I

[0019] in which

[0020] A¹ and A² are each, independently of one another and in the casewhere A¹ occurs a number of times, these too independently of oneanother:

[0021] a) 1,4-cyclohexylene or trans-1,4-cyclohexenylene, in which, inaddition, one or more non-adjacent CH₂ groups may be replaced by Oand/or S,

[0022] b) 1,4-phenylene, in which, in addition, one or two CH groups maybe replaced by N,

[0023] c) 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl,naphthalene-2,6-diyl, decahydronaphthalene-3,6-diyl or1,2,3,4-tetrahydronaphthalene-2,6-diyl,

[0024]  in which (a) and (b) may be monosubstituted or disubstituted byF atoms,

[0025] Z¹ and Z² are each, independently of one another and in the casewhere Z¹ occurs a number of times, these too independently of oneanother:

[0026] —CO—O—, —O—CO—, —CO—CH₂—, —CH₂—CO—, —CH₂O—, —OCH₂—, —CH₂CH₂—,—CH═CH—, —C≡C— or a single bond, or one of the groups Z¹ and Z² is

[0027] —(CH₂)₄—, —(CH₂)₃CO—, —(CH₂)₂—O—CO—, —(CH₂)₂—(CO—O)—,CH═CH—CH₂CH₂—, —CH₂—CH₂—CH═CH— or —CH₂—CH═CH—CH₂—,

[0028] R is H, alkyl or alkenyl having 1 or 2 to 15 carbon atomsrespectively, which are unsubstituted, monosubstituted by CN or CF₃ ormonosubstituted or polysubstituted by halogen, in particular F or Cl,where, in addition, one or more CH₂ groups in these radicals may bereplaced, independently of one another, by —O—, —S—,

[0029]  —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that no two Oatoms are bonded directly to one another,

[0030] or

[0031] CN, F, Cl or COOR′, if appropriate also OH,

[0032] R′ is H or R, where CN, F, OH and COOR′ are excluded,

[0033] n is 0, 1 or 2,

[0034] m is 0 or 1,

[0035] o is 1, 2 or 3,

[0036] n+m+o is 2, 3 or 4,

[0037] X and Y are each, independently of one another and in the casewhere X and/or Y occur a number of times, these too independently of oneanother, are F, Cl, COOR′, NO₂ or CN, where X is preferably F and Y ispreferably CN,

[0038] p₁, p₂,

[0039] q₁ and q₂ are each 0, 1, 2, 3 or 4,

[0040] one of q₁ and q₂, preferably q₂, is preferably 1

[0041] where

[0042] o is 1 and q₂ is 0

[0043] p2 is 3 or 4

[0044] where

[0045] o is 1 and q₂ is 1, 2 or 3

[0046] p₂+q₂ is 1, 2, 3 or 4

[0047] preferably q₂ is 1 and p₂ is 1 or 2

[0048] where

[0049] o is 2 or 3

[0050] p₁+p₂+

[0051] q₁+q₂ is 1, 2, 3, 4, 5, 6, 7, or 8,

[0052] where

[0053] q₁+q₂ is 1 or 2

[0054] p₁+p₂ is 1, 2, 3 or 4

[0055] preferably

[0056] q₁+q₂ is 1 and p₁+p₂ is 1 or 2

[0057] where

[0058] q₁+q₂ is 0

[0059] p₁+p₂ is 3, 4, 5, 6, 7 or 8

[0060] preferably 6, 7 or 8

[0061] Preference is given to compounds of the formula Ia

[0062] in which

[0063] in which

[0064] A¹ and A² are each, independently of one another and in the casewhere A¹ occurs a number of times, these too independently of oneanother:

[0065] a) 1,4-cyclohexylene or trans-1,4-cyclohexenylene, in which, inaddition, one or more non-adjacent CH₂ groups may be replaced by Oand/or S,

[0066] b) 1,4-phenylene, in which, in addition, one or two CH groups maybe replaced by N,

[0067] c) 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl,naphthalene-2,6-diyl, decahydronaphthalene-3,6-diyl or1,2,3,4-tetrahydronaphthalene-2,6-diyl,

[0068]  in which (a) and (b) may be monosubstituted or disubstituted byF atoms,

[0069] Z¹ and Z² are each, independently of one another and in the casewhere Z¹ occurs a number of times, these too independently of oneanother:

[0070] —CO—O—, —O—CO—, —CO—CH₂—, —CH₂—CO—, —CH₂O—, —OCH₂—, —CH₂CH₂—,—CH═CH—, —C≡C— or a single bond, or one of the groups Z¹ and Z² is

[0071] —(CH₂)₄—, —(CH₂)₃CO—, —(CH₂)₂—O—CO—, —(CH₂)₂—(CO—O)—,CH═CH—CH₂CH₂—, —CH₂—CH₂— CH═CH— or —CH₂—CH═CH—CH₂—,

[0072] R is H, alkyl or alkenyl having 1 or 2 to 15 carbon atomsrespectively, which are unsubstituted, monosubstituted by CN or CF₃ ormonosubstituted or polysubstituted by halogen, in particular F or Cl,where, in addition, one or more CH₂ groups in these radicals may bereplaced, independently of one another, by —O—, —S—,

[0073]  —CO—, —CO—C—, —O—CO— or —O—CO—O— in such a way that no two Oatoms are bonded directly to one another,

[0074] or

[0075] CN, F, Cl or COOR′, if appropriate also OH,

[0076] R′ is H or R, where CN, F and COOR′ are excluded,

[0077] n is 0, 1 or 2,

[0078] m is 0 or 1,

[0079] o is 1, 2 or 3,

[0080] n+m+o is 2, 3 or 4,

[0081] X and Y are each, independently of one another and in the casewhere X and/or Y occur a number of times, these too independently of oneanother, are F, Cl, COOR′, NO₂ or CN, where X is preferably F and Y ispreferably CN,

[0082] one of q₁ and q₂, preferably q₂, is preferably 1

[0083] where

[0084] o is 1 and q₂ is 0

[0085] p2 is 3 or 4

[0086] where

[0087] o is 1 and q₂ is 1, 2 or 3

[0088] p₂+q₂ is 1, 2, 3 or 4

[0089] preferably q₂ is 1 and p₂ is 1 or 2

[0090] where

[0091] o is 2 or 3

[0092] p₁+p₂+

[0093] q₁+q₂ is 1, 2, 3, 4, 5, 6, 7, or 8,

[0094] where

[0095] q₁+q₂ is 1 or 2

[0096] p₁+p₂ is 1, 2, 3 or 4

[0097] preferably

[0098] q₁+q₂ is 1 and p₁+p₂ is 1 or 2

[0099] where

[0100] q₁+q₂ is 0

[0101] p₁+p₂ is 3, 4, 5, 6, 7 or 8

[0102] preferably 6, 7 or 8

[0103] In formula I and Ia, the hydroxy-group may be located at theterminal phenyl group and/or other phenyl groups, where present.Compounds with hydroxy groups only on the terminal phenyl group are ofsubformulae I′ and Ia′:

[0104] Preference is furthermore given to compounds of the formula Ib

[0105] in which

[0106] A¹ and A² are each, independently of one another and in the casewhere A¹ occurs a number of times, these too independently of oneanother:

[0107] a) 1,4-cyclohexylene or trans-1,4-cyclohexenylene, in which, inaddition, one or more non-adjacent CH₂ groups may be replaced by Oand/or S,

[0108] b) 1,4-phenylene, in which, in addition, one or two CH groups maybe replaced by N,

[0109] c) 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl,naphthalene-2,6-diyl, decahydronaphthalene-3,6-diyl or1,2,3,4-tetrahydronaphthalene-2,6-diyl,

[0110]  in which (a) and (b) may be monosubstituted or disubstituted byF atoms,

[0111] Z¹ and Z² are each, independently of one another and in the casewhere Z¹ occurs a number of times, these too independently of oneanother:

[0112] —CO—O—, —O—CO—, —CO—CH₂—, —CH₂—CO—, —CH₂O—, —OCH₂—, —CH₂CH₂—,—CH═CH—, —C≡C— or a single bond, or one of the groups Z¹ and Z² is

[0113] —(CH₂)₄—, —(CH₂)₃CO—, —(CH₂)₂—O—CO—, —(CH₂)₂—(CO—O)—,CH═CH—CH₂CH₂—, —CH₂—CH₂— CH═CH— or —CH₂—CH═CH—CH₂—,

[0114] R is H, alkyl or alkenyl having 1 or 2 to 15 carbon atomsrespectively, which are unsubstituted, monosubstituted by CN or CF₃ ormonosubstituted or polysubstituted by halogen, in particular F or Cl,where, in addition, one or more CH₂ groups in these radicals may bereplaced, independently of one another, by —O—, —S—,

[0115]  —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that no two Oatoms are bonded directly to one another,

[0116] or

[0117] CN, F, Cl or COOR′, if appropriate also OH,

[0118] R′ is H or R, where CN, F and COOR′ are excluded,

[0119] n is 0, 1 or 2,

[0120] m is 0 or 1,

[0121] o is 1, 2 or 3,

[0122] n+m+o is 2, 3 or 4,

[0123] X and Y are each, independently of one another and in the casewhere X and/or Y occur a number of times, these too independently of oneanother, are F, Cl, COOR′, NO₂ or CN, where X is preferably F and Y ispreferably CN,

[0124] p and q are 0, 1, 2 or 3,

[0125] p+q is 1, 2, 3, 4, 5, 6, 7 or 8,

[0126] q is preferably 0 or 1,

[0127] and, in the case where q=0,

[0128] p is 3, 4, 5, 6, 7 or 8,

[0129] in the case where q=1,

[0130] p is 1, 2, 3, 4, 5, 6, or 7,

[0131] and X is preferably F and Y is preferably CN.

[0132] In the formula Ia,

[0133] is preferably, where o=1,

[0134] in which X_(p1), X_(p2), Y_(q1) and Y_(q2) are as defined aboveunder the formula Ia.

[0135] In the formula Ib,

[0136] in which

[0137] X is F, Cl or COOR′, preferably F,

[0138] Y is CN or NO₂, preferably CN

[0139] p and q are each 0, 1, 2 or 3, and

[0140] p+q is 2 or 3, and, in the case where R═CN, F, Cl, OH or COOR′,is alternatively 1.

[0141] In the formula Ib,

[0142] in which X_(p) and Y_(q) are as defined under formula Ib.

[0143] X is preferably F. Y is preferably CN.

[0144] Particular preference is given to compounds of the subformulaeIa-1 to Ia-6

[0145] in which

[0146] R, A¹, A², Z¹, Z², n, m, X and Y are as defined above under theformula Ia. Y is preferably CN, X is preferably F, n is preferably 0,and R is preferably CN, F or OH.

[0147] Particular preference is given to compounds of the subformulaeIb-1 to Ib-5.

[0148] in which R, A¹, Z¹, n, X and Y are as defined above under theformula Ib. Y is preferably CN, and X is preferably F.

[0149] Particular preference is given to compounds of the subformulaeIa-3a, Ia-4a, Ia-5a and Ia-6a:

[0150] in which R, A² and m are as defined above under the formula I.

[0151] In the subformulae Ia-3a, Ia-4a, Ia-5a and Ia-6a,

[0152] Particular preference is given to compounds of the subformulaeIb-1a, Ib-2a and Ib-2b:

[0153] in which R is as defined above under the formula I.

[0154] In the subformulae Ib-1a, 1b-2a and Ib-2b,

[0155] Very particular preference is given to compounds of the formulaIa-1-1

[0156] in which R is CN, F, Cl or OH, preferably F.

[0157] Very particular preference is furthermore given to compounds ofthe formulae Ib-1a1, 1b-2a1 and 1b-3a1:

[0158] in which R is as defined above under the formula I. In the caseof compounds of the formula Ib-1a1, R is particularly preferably alkylor alkenyl having 1 to 7 carbon atoms, very particularly preferablyethyl, propyl, butyl or pentyl. In the case of compounds of the formulaIb-2a1, R is particularly preferably alkyl or alkoxy having 1 to 7carbon atoms, very particularly preferably methoxy, ethoxy, propoxy orbutoxy.

[0159] Besides liquid-crystal mixtures comprising the abovementionedphenol compounds, the present application also relates to thesecompounds themselves, as long as they were still unknown. Theseliquid-crystal mixtures are particularly suitable for use inliquid-crystal displays, especially for STN and IPS displays.

[0160] Liquid-crystal mixtures for STN displays and especially theliquid-crystal display mixture concepts which are favourable for suchdisplays are described in EP 0 394 417, and those of AMD displays aredescribed in EP 0 394 419.

[0161] Liquid-crystal mixtures for TN displays in accordance with thepresent application preferably comprise terminally cyano-substitutedcompounds as dielectrically positive compounds. They very particularlypreferably comprise compounds of the formula II

[0162] in which

[0163] A³, in the case where A³ occurs a number of times, theseindependently of one another, is

[0164] a) 1,4-cyclohexylene or trans-1,4-cyclohexenylene, in which, inaddition, one or more non-adjacent CH₂ groups may be replaced by Oand/or S,

[0165] b) 1,4-phenylene in which, in addition, one or two CH groups maybe replaced by N,

[0166] c) 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl,naphthalene-2,6-diyl, decahydronaphthalene-3,6-diyl or1,2,3,4-tetrahydronaphthalene-2,6-diyl,

[0167]  in which (a) and (b) may be monosubstituted or disubstituted byF atoms,

[0168] Z³ in the case where Z³ occurs a number of times, theseindependently of one another, is

[0169] —CO—O—, —O—CO—, —CO—CH₂—, —CH₂—CO—, —CH₂O—, —OCH₂—, —CH₂CH₂—,—CH═CH—, —C≡C— or a single bond,

[0170] R³ is H, alkyl or alkenyl having 1 or 2 to 15 carbon atomsrespectively, which are unsubstituted, monosubstituted by CN or CF₃ ormonosubstituted or polysubstituted by halogen, in particular F or Cl,where, in addition, one or more CH₂ groups in these radicals may bereplaced, independently of one another, by —O—, —S—,

[0171]  —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that no two Oatoms are directly bonded to one another,

[0172] r is 1 or 2,

[0173] X³ is H or F, and

[0174] Y³ is H or F.

[0175] The mixtures preferably comprise compounds of the formula II-1and, if desired, II-2

[0176] in which R³ is as defined above under the formula II and ispreferably n-alkyl or 1-E-alkenyl.

[0177] Preference is given to liquid-crystal mixtures comprisingdielectrically positive compounds of the formula II where r=1,A³=1,4-cyclohexylene, X³=H, Z³ is a single bond, Y³=H, and R=alkyl oralkenyl having 1 or 2 to 7 carbon atoms, respectively. In particular,the liquid-crystal mixtures are based on these compounds, at leastregarding their dielectrically positive constituents. This applies inparticular to the mixtures for TN displays and also for STN displays.Compounds of the formula II where r=1, A³=1,4-phenylene, Z³=—CO—O—,X³=H, Y³=F and R=alkyl or alkenyl having 1 or 2 to 7 carbon atomsrespectively are also present in preferred liquid-crystal mixtures.However, the concentration of the compounds of the formula II-2 isgenerally significantly lower than that of the compounds of the formulaII-1. The threshold voltage of the preferred liquid-crystal mixtures is,preferably in TN cells, preferably in the range from 1.5 V to 4.0 V,very particularly preferably from 1.8 to 3.5 V, especially preferablyfrom 2.0 to 3.0 V.

[0178] With regard to the liquid-crystal mixtures and liquid-crystalmixture concepts for STN and AMD displays, EP 394417 and EP 394419mentioned above are incorporated into the present application by way ofreference.

[0179] For TN and STN displays, particularly preferred liquid-crystalmixtures in accordance with the present application are those comprisingterminally cyano-substituted compounds, such as, for example,benzonitriles and benzonitriles which are monosubstituted ordisubstituted by fluorine in the ortho position to the cyano group.

[0180] The liquid-crystal mixtures for STN displays in accordance withthe present application may also comprise terminally fluorinatedcompounds and/or compounds carrying terminally fluorine-containingsubstituents.

[0181] Liquid-crystal mixtures for STN displays according to the presentapplication very particularly preferably comprise compounds containingalkenyl side chains. These compounds can be either dielectricallypositive and contain an alkenyl side chain or dielectrically neutral. Inthe latter case, the compounds can contain one or two alkenyl sidechains.

[0182] In liquid-crystal mixtures for AMD displays, preference is givenin the present application to terminally fluorinated compounds and/orcompounds carrying terminally fluorine-containing substituents. Thesemixtures preferably comprise at most 10%, particularly preferably atmost 5%, very particularly preferably at most 1%, of cyano-substitutedcompounds. In particular, such mixtures comprise no cyano-substitutedcompounds apart from, if desired, those of the formula I and itssubformulae.

[0183] The compounds according to the invention or the compounds usedaccording to the invention are particularly preferably used in mixturesfor IPS displays. For such IPS displays, preference is given in thepresent application to liquid-crystal mixtures comprising terminallyfluorinated compounds, like the mixtures for AMD displays. However, themixtures for IPS displays additionally comprise terminallycyano-substituted compounds. The concentration of these terminallycyano-substituted compounds can vary within broad limits.

[0184] In particular, it is not restricted to a low upper limit as inthe case of the mixtures for AMD displays. Typically, from 1 to 50% ofcyano-substituted compounds can be employed. Preference is given tomixtures comprising from 5 to 35%, particularly preferably from 7 to25%, of cyano-substituted compounds.

[0185] Liquid-crystal mixtures for IPS displays and mixture concepts forthese liquid-crystal displays are described in GB 23 10 669, EP 0 807153, DE 19528104, DE 19528107, EP 0 768 359, DE 19 611 096 and DE 19 625100. These seven patent applications are incorporated herein by way ofreference regarding liquid-crystal mixtures for IPS displays andregarding their mixture concepts.

[0186] The structure of the TN, STN, AMD and IPS displays is known orfollows known rules. The terms TN, STN, AMD and IPS here are broadlydrawn and also cover typical modifications of displays of these types.

[0187] The structure of STN displays-is described in EP 0 098 070, EP 0131 216 and EP 0 260 450 and that of IPS displays is described, interalia, in U.S. Pat. No. 5,576,867 and EP 0 588 568. Regarding theconstruction of STN and IPS displays, the abovementioned five patentapplications are incorporated herein by way of reference.

[0188] Synthesis of the Compounds

[0189] R, A¹, Z¹, A², Z², n and m are as defined above under the formulaI.

[0190] The compounds according to the invention are added to theliquid-crystal mixtures in concentrations of from 1 ppm to typically50,000 ppm, preferably 10 ppm to 10,000 ppm, particularly preferably 50ppm to 5000 ppm (based on proportions by weight).

[0191] The physical properties are determined, unless describedotherwise, as described in “Physical Properties of Liquid Crystals”,Merck KGaA, Ed. W. Becker, Nov. 1997.

[0192] The specific resistances (abbreviated to SR) of theliquid-crystal mixtures per se are determined as described in G. Weberet al. “Liquid crystals for active matrix displays”, Liquid crystals,1989, Vol. 5, No. 5, pages 1381-1388, using 1 ml in a stainless-steelmeasurement cell.

[0193] The specific resistances of the liquid-crystal mixtures arefurthermore determined in glass test cells (SRZ). These test cellsconsist, like liquid-crystal display cells, of two glass plates bondedparallel to one another at a distance from one another. The cells areproduced in the test cell manufacturing facility at Merck KGaA. Theyconsist of alkali-free glass with indium tin oxide (ITO) electrodes withno alignment layer. The layer thickness is 20 μm. The cells have twofill openings on opposite sides. The electrodes are circular, eachhaving an area of 1 cm². For screening against electromagneticradiation, the cells are provided with a ring-shaped protectiveelectrode.

[0194] The specific resistance of the liquid-crystal mixtures in testcells (SRZ) is, like the specific resistance as such (SR), measuredusing a sensitive electrometer (Keithley 6517 High-resistance system).The Keithley 617 is used when measuring the specific resistance as such(SR). The filling of the dried cells and the measurement of theresistance all take place in a system of glove boxes connected by airlocks. A stream of dry nitrogen passes continuously through these gloveboxes. The relative atmospheric humidity inside the boxes containing thecells must be lower than 10%, in particular during filling andmeasurement of the cells.

[0195] The resistance is measured in an aluminium housing in which thetest cells are held at 20° C. by means of two brass blocks with water asheating medium.

[0196] For shielding against stray electromagnetic fields, the guardring of the measurement cells is earthed together with the aluminiumhousing.

[0197] The earthing of the guard ring is implemented separately for eachof the measurement lines and together with the earthing of the aluminiumhousing in such a way that earth loops do not form.

[0198] The capacitance of the cells in the discharged state isdetermined separately before filling. It is typically ε_(empty)=45 pF.

[0199] In order to determine the resistance, the current is measured ata constant direct voltage of 20 V. To this end, the test cell isaddressed as follows. After a waiting time of 20 s, four 20 V pulses ofalternating polarity are applied. All four pulses are 20 s in width andseparated from one another by a 0 V interval of 20 s. The last of thefour pulses is followed by a 0 V interval of 180 s, and then 20 V withthe polarity of the first pulse is again applied for 180 s. At the endof this final pulse, ten measurement readings are taken within 5 s;these give the average resistance.

[0200] The heating is carried out in another of the three glove boxes.After the heating, typically at 120° C. for 1 hour, the cell is allowedto cool and then transferred back into the glove box together with thetemperature-maintained cell holder.

[0201] The resultant specific resistance (SR) values of the finishedliquid-crystal mixtures are from 10⁹ Ωcm to 10¹³ Ωcm, depending on thearea of application and the specification. Liquid-crystal mixtures forTN and STN displays preferably have specific resistance values of from10¹⁰ Ωcm to 10¹² Ωcm, especially from 5·10¹⁰ Ωcm to 2·10¹¹ Ωcm, andliquid-crystal mixtures for IPS displays preferably have specificresistance values of from 5·10¹¹ Ωcm to 8·10¹² Ωcm, preferably from 10¹²Ωcm to 3·10¹² Ωcm.

[0202] Concentrations of dopant in the range from 10 to 1000 ppm areparticularly preferably employed and specific resistances in cells (SRZ)of from 5·10¹¹ Ωcm to 1·10¹³ Ωcm are obtained. Relatively smallconcentrations of dopant are preferred.

[0203] The physical properties given above and below in this applicationapply to and are given for a temperature of 20° C., unless explicitlystated otherwise.

[0204] The “Voltage Holding Ratio” (VHR or HR) is measured as describedin “Physical Properties of Liquid Crystals”, VIII, Voltage HoldingRatio, Merck KGaA, Ed. W. Becker, Nov. 1997. Test cells having a layerthickness of about 5 μm and an electrode area of 1 cm² were used. Thevoltage holding ratio was determined using a commercially availableinstrument from Antonic-Melchers, Germany. The measurement voltage was 1V.

[0205] The electrooptical properties, in particular the threshold valuesand the steepness values, were determined in test cells produced byMerck KGaA. The threshold voltage V_(th) or V₁₀ was determined for a 10%change in the relative contrast, i.e. between a non-addressed cell and acell addressed to full saturation. The steepness was determined asV₉₀/V₁₀.

[0206] Cells having a twist angle of 90° were used. The cells wereoperated in normally white mode. The liquid-crystal mixtures were notdoped with chiral dopants. The layer thickness of the cells was selectedso that the optical retardation was 0.50 μm.

[0207] The measuring instruments used were a commercially availableinstrument from Otsuka, Japan, and a matched measuring instrument fromMerck KGaA.

[0208] The present application also relates to a method of achieving andadjusting a certain specific resistance in liquid-crystal mixtures bymeans of acidic mesogenic compounds, preferably by means ofappropriately substituted phenols.

[0209] The present application relates to a method of adjusting theresistance of liquid-crystal mixtures to a desired value by usingacidic, mesogenic or liquid crystal-like compounds. Particularpreference is given to acidic compounds having a sufficiently highpK_(a) value. The minimum values for pK_(a) depend on the resistance andthe polarity (in particular the dielectric anisotropy) of theliquid-crystal mixture. In the case of liquid-crystal mixtures havinglow resistance values, for example from 10¹⁰ to 10¹¹ Ωcm, it isnecessary to use acidic compounds having larger pK_(a) values than inthe case of the liquid-crystal mixtures having greater resistancevalues.

[0210] The pK_(a) value of the compound(s) employed (preferably phenolcompounds) should be at least as large as that of the compoundCCU-3-CN.OH—F from Example 2c (Substance Example 5) (cf. Example 10).The pK_(a) value of the compound(s) employed is preferably greater thanthat of CCU-3-CN—OH—F.

[0211] The compounds preferably to be employed carry more than 3fluorine substituents on the OH-substituted ring or, in the case ofbiphenyl compounds, more than 2 fluorine substituents on each ring.

[0212] The compounds particularly preferably carry 7, 8 or 9 fluorinesubstituents so long as they contain an OH-substituted biphenyl system.The compounds particularly preferably contain one CN and at least one Fsubstitution on the OH-substituted benzene ring. The CN group ispreferably in the ortho- or para-position to the OH, particularlypreferably in the ortho-position.

[0213] It is furthermore necessary to use compounds whose vapourpressure is wherever possible not excessively high. Preference is givento compounds whose vapour pressure is not higher than that of theliquid-crystal compounds usually employed, for example

[0214] Particular preference is given to compounds whose vapour pressureis not higher than that of

[0215] Particular preference is given to acidic phenols, especiallycompounds of the formula I and its subformulae of the presentapplication.

[0216] The construction of the liquid-crystal display elements accordingto the invention from polarizers, electrode baseplates and electrodeshaving a surface treatment such that the preferential alignment(director) of the liquid-crystal molecules adjacent thereto is usuallytwisted from one electrode to another by a value of about 90° (inparticular of about 80° to 110°) or from 160° to 720°, corresponds tothe usual design for display elements of this type. The term usualdesign is broadly drawn here and also covers all derivatives andmodifications of the TN and STN cell, in particular also matrix displayelements and display elements containing additional magnets. The surfacetilt angles at the two outer plates can be identical or different.Identical tilt angles are preferred. Preferred TN displays have tiltangles between the long axis of the molecules at the surface of theouter plates and the outer plates of from 0° to 7°, preferably from0.01° to 5°, in particular from 0.1 to 2°. In STN displays, the tiltangle is from 1° to 30°, preferably from 1° to 12°, in particular from3° to 8°.

[0217] The twist angle of the TN mixture in the cell has a value of from22.5° to 170°, preferably from 45° to 130°, in particular from 80° to115°. In a display, the twist angle of the STN mixture from alignmentlayer to alignment layer has a value of from 100° to 600°, preferablyfrom 170° to 270°, in particular from 180° to 250°.

[0218] IPS displays of the present application preferably have anuntwisted starting state and are twisted about an axis perpendicular tothe substrates by the component of the electric field parallel to thesubstrates. The tilt angle is preferably in the range from 0° or justabove 0° (for example 0.1°) to 15°, particularly preferably from 0.2 to6°.

[0219] The liquid-crystal mixtures which can be used in accordance withthe invention are prepared in a manner conventional per se. In general,the desired amount of the components used in lesser amount are dissolvedin the components making up the principal constituent, expediently atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and, after mixing, to remove the solvent again, for example bydistillation.

[0220] The liquid-crystal mixtures can also contain further additivesknown to the person skilled in the art and described in the literature.For example, 0-15% of pleochroic dyes can be added.

[0221] The examples below are intended to illustrate the inventionwithout representing a limitation.

[0222] The following abbreviations are used:

[0223] T(S,N) smectic-nematic phase transition temperature,

[0224] T(N,I) nematic-isotropic phase transition temperature,

[0225] cl.p. clearing point

[0226] t_(on) time from switching on until 90% of the maximum contrastis reached

[0227] t_(off) time from switching off until 10% of the maximum contrastis reached

[0228] V₉₀/V₁₀ steepness $\frac{t_{on} + t_{off}}{2}$

[0229] Δε dielectric anisotropy

[0230] Δn optical anisotropy

[0231] SR specific resistance in bulk

[0232] SRZ specific resistance in test cells

[0233] Above and below, all temperatures are given in 0° C., thepercentages are percent by weight, and the values for the physicalproperties relate to 20° C., unless otherwise specified.

[0234] In the present application and in the examples below, thestructures of the liquid-crystal compounds are indicated by acronyms,the transformation into chemical formulae taking place in accordancewith Tables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1)are straight-chain alkyl radicals having n or m carbon atomsrespectively. The alkenyl radicals have the trans-configuration. Thecoding in Table B is self-evident. In Table A, only the acronym for theparent structure is given. In individual cases, the acronym for theparent structure is followed, separated by a hyphen, by a code for thesubstituents R¹, R², L¹, L² and L³. Code for R¹, R^(2,) L¹, L², L³ R¹ R²L¹ L² L³ nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H H nOm C_(n)H_(2n+1)OC_(m)H_(2m+1) H H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H H nC_(n)H_(2n+1) CN H H H nN.F C_(n)H_(2n+1) CN H H F nF C_(n)H_(2n+1) F HH H nOF OC_(n)H_(2n+1) F H H H nCl C_(n)H_(2n+1) Cl H H H nF.FC_(n)H_(2n+1) F H H F nmF C_(n)H_(2n+1) C_(m)H_(2m+1) F H H nCF₃C_(n)H_(2n+1) CF₃ H H H nOCF₃ C_(n)H_(2n+1) OCF₃ H H H nOCF₂C_(n)H_(2n+1) OCHF₂ H H H nS C_(n)H_(2n+1) NCS H H H rVsNC_(n)H_(2n+1)—CH═CH— CN H H H C₅H_(2s)— rEsN C_(n)H_(2n+1)—O—C₂H_(2s)—CN H H H nNF C_(n)H_(2n+1) CN H F H nAm C_(n)H_(2n+1) COOC_(m)H_(2m+1) HH H nF.Cl C_(n)H_(2n+1) Cl H H F n-Vm C_(n)H_(2n+1) —CH═CH—C_(m)H_(2m+1)H H H nV-Vm C_(n)H_(2n+1)—CH═CH— —CH═CH—C_(m)H_(2m+1) H H H nV-mC_(n)H_(2n+1)—CH═CH— —C_(m)H_(2m+1) H H H nV-N C_(n)H_(2n+1)—CH═CH— —CNH H H nV-F C_(n)H_(2n+1)—CH═CH— F H H H nV-F.F C_(n)H_(2n+1)—CH═CH— F HH F n-2Vm C_(n)H_(2n+1) C_(m)H_(2m+1)—CH═CH—CH₂CH₂— H H H

[0235] The TN, STN and IPS displays preferably contain liquid-crystalmixtures composed of one or more compounds from Tables A and B.

[0236] Table A:

[0237] (L¹, L², L³: H or F) TABLE A (L¹, L², L³: H or F)

BCH

CBC

CCH

CCP

CPTP

D

ECCP

EPCH

HP

ME

CCPC

CH

PCH

PTP

CP

PDX

CCZU-n-F

CC-1V-V1

[0238] TABLE B

K3n

M3n

BCH-n.FX

PYP-nF

Inm

C-nm

C15

S-811

CP-nmF

CB15

CC-n-V

CBC-nmF

CC-n-V1

CCG-V-F

[0239] The examples below are intended to illustrate the inventionwithout representing a limitation. Above and below, percentages arepercent by weight. All temperatures are given in degrees Celsius. m.p.denotes melting point, cl.p.=clearing point. Furthermore, C=crystallinestate, N=nematic phase, S=smectic phase and I=isotropic phase. The databetween these symbols are the transition temperatures. Δn denotesoptical anistropy (589 nm, 20° C.). Δε denotes the dielectric anisotropy(1 kHz, 20° C.).

EXAMPLES Example 1 (Substance Example)

[0240] Compound 1

[0241] 4.1 g of2,6-difluoro-4-(4-n-propyl-trans-cyclo-hexyl)benzonitrile

[0242] were dissolved in 150 ml of THF, and the solution was introducedinto a 250 ml four-neck flask. 1.75 g of potassium tert-butoxide weredissolved in 30 ml of THF, and the solution was slowly added dropwisewith stirring at such a rate that the temperature did not exceed 30° C.The reddish reaction mixture was stirred for 12 hours, then hydrolysedusing about 40 ml of conc. HCl, whereupon it turned a yellow colour.During the hydrolysis, it was ensured that the temperature did notexceed 30° C. The mixture was extracted 3 times with 50 ml ofmethyltertiarybutyl ether (MTB) in each case. The organic phase was thenwashed twice with 50 ml of H₂O, dried and evaporated, and the residuewas purified by chromatography on silica gel in MTB/Lexane (1:1) via afrit. The fractions were then evaporated, giving 2.5 g of solid, whichwas recrystallized from n-heptane, giving 1.7 g of the compound2-cyano-3-fluoro-5-(4-n-phenyl-trans-cyclohexyl) phenol

[0243] abbreviated to PCH-3N.F.OH.

[0244] The melting point was 138.6° C.

[0245] The following were prepared analogously:

[0246] Compounds 2 to 7

Example 2 (Substance Example 2)

[0247] Compound No. 8

[0248] was prepared analogously to Compound 1 in Example 1 using thecorresponding starting compound, in accordance with the followingscheme.

[0249] R, A¹, Z¹, A², Z², m and n are as defined above under the formulaI.

Example 2a (Substance Example 3)

[0250] Compound No. 9

[0251] was prepared.

[0252] The physical properties are:

[0253] phase sequence C 71° C. N 83.4° I

Example 2b (Substance Example 4)

[0254] Compound No. 10

[0255] was prepared.

[0256] C 112° C. I

Example 2c (Substance Example 5)

[0257] Compound No. 11 (CCP-3-CN.OH-F.F.F. for Short)

[0258] was prepared.

[0259] C 229° C. I

Example 2d (Substance Example 6)

[0260] The Compound

[0261] was prepared.

[0262] K 94° C. I

Example 2e (Substance Example 7)

[0263] The Compound

[0264] was prepared.

[0265] C 146° C. I

Example 3 (Use Example 1)

[0266] 1000 ppm or 10,000 ppm (by weight) of the compound PLH-3N.F.OHfrom Example 1 were added to a liquid-crystal mixture A-0.

[0267] The composition and physical properties of the liquid-crystalmixture A-0 are shown in the table below (Table 1). The concentrationdata for the individual compounds, like all concentration data in thisapplication, are by weight, unless explicitly stated otherwise. TABLE 1Mixture A-0 Composition Substance Concentration/% PCH-3 13.75 PCH-4 2.75PCH-301 17.00 PCH-302 16.75 PCH-304 13.00 CH-33 3.25 CH-35 3.25 CH-433.25 CH-45 3.00 CBC-33F 4.75 CBC-53F 5.50 CBC-55F 3.75 CBC-33 4.00CBC-53 6.00

[0268] Physical Properties T (N, I) = 96° C. Δn (589 nm) =  0.108 Δε (1kHz) =  3.1 V₁₀ =  2.8 V

[0269] The specific resistance of mixtures A-0, A-1000 and A-10,000 weremeasured. The results are shown in the table below (Table 2): TABLE 2Specific resistance of the liquid-crystal mixture A containingPCH-3N.F.OH Mixture c (dopant)/ppm SR/Ωcm A-0 0 1.4 · 10¹³ AP-1000 10001.1 · 10¹¹ AP-10,000 10,000 1.0 · 10¹¹/2.0 · 10¹⁰

Example 4 (Use Example 2)

[0270] 1000 ppm (by weight) of compound PCH-3N.F.OH from Example 1 wereadded to the liquid-crystal mixture A-0 from Example 3, and theresultant mixture A-1000 was divided into 2 parts, one of which wasfiltered through a Millipore filter (Teflon 40 μm) (A-1000 F).

[0271] The properties of the three mixtures A-0, A-1000 and A-1000 Fthus obtained and of mixture A-10,000 from Example 3 were measured indifferent test cells. These test cells were produced in the test cellfabrication facility at Merck KGaA. The test cells all had a layerthickness of 6.0 μm.

[0272] Test cells having five different alignment layers were used.

[0273] These alignment layers are shown in the table below (Table 3).TABLE 3 Summary of the alignment layers used Name Type Trade nameManufacturer PI-1 Polyimide AL-1051 Japan Synthetic Rubber PI-2Polyimide CU-1523 DuPont-Merck PI-3 Polyimide CU-2062 DuPont-Merck PI-4Polyimide LX-1400 Hitachi PI-5 Polyimide SE-130 Nissan Chemicals

[0274] The properties of the three mixtures in the test cells having thefive different alignment layers are shown in the table below (Table 4).All the values given are, as in the entire application, averages of atleast two individual measurements, unless otherwise stated. TABLE 4Physical properties of mixture A containing PCH-3N.F.OH with differentalignment layers Mixture A-0 AP-1000 AP-1000 F AP-10,000 c(dop.)/ppm 01000/1 1000 10,000 SR/Ωcm 1.4 · 10¹³ 1 · 10¹¹ 3.9 · 10¹¹ 20 · 10¹⁰Polyimide HR (20° C.)/% PI-1 99.5 97.7 97.0 50 PI-2 95.5 94.1 94.3 57PI-3 85.9 76 87.5 57 PI-4 92.0 98.3 91.6 86.7 PI-5 98.5 92.6 97.9 45Polyimide HR(100° C.)/% PI-1 93.0 n.d. n.d. n.d. PI-2 67 n.d. n.d. n.d.PI-3 59 n.d. n.d. n.d. PI-4 61 n.d. n.d. n.d. PI-5 76 n.d. n.d. n.d.Polyimide V₁₀/V PI1 3.0 3.0 3.0 2.6 PI2 3.0 3.1 2.9 2.9 PI3 2.9 2.8 2.92.8 PI4 2.8 2.8 2.8 2.8 PI5 2.8 2.8 2.9 2.7 Polyimide (V₉₀/V₁₀-1)/% PI143 44 43 43 PI2 47 48 42 49 PI3 47 47 44 45 PI4 49 44 44 45 PI5 45 47 4647

Example 5 (Use Example 3)

[0275] Liquid-crystal mixture B-0 was prepared and divided into fourparts. Compound PCH-3N.F.OH from Example 1 was added to three parts inconcentrations of 100 ppm, 1000 ppm and 10,000 ppm.

[0276] The resultant three mixtures of different concentrations B-100,B-1000 and B-10,000 and the original mixture B-0, containing noPCH-3N.F.OH, were investigated with respect to their physicalproperties, in particular with respect to their specific resistance. Theresults are shown in the table below (Table 6).

[0277] The liquid-crystal mixture B-0 has the composition and physicalproperties shown in the table below (Table 5). TABLE 5 Mixture B-0Composition Compound Concentration/% CCP-2F.F.F 6.00 CCP-3F.F.F 9.00CCP-5F.F.F 2.00 CCP-2OCF3 8.00 CCP-3OCF3 6.00 CCP-4OCF3 4.00 CCP-5OCF37.00 PDX-3 2.00 PDX-4 8.00 PDX-5 8.00 CCZU-2-F 3.00 CCZU-3-F 16.00CCZU-5-F 3.00 CCH-301 6.00 CCH-303 6.00 CCH-501 6.00

[0278] Physical Properties T(N,I) = 72° C. Δn (589 nm) =  0.075 Δε (1kHz) = 10.2 V₀ (Frederick No.) =  1.0 V

[0279] The specific resistance values of mixtures B-0 to B-10,000 areshown in the table below (Table 6). TABLE 6 Specific resistance ofmixture B containing PCH-3N.F.OH c(dop.)/ SR SRZ(20° C.) SRZ (120° C., 1h, Mixture ppm (20° C.)/Ωcm /Ωcm 20° C.)/Ωcm B-0 0 2.5 · 10¹³ 3.0 · 10¹³1.7 · 10¹³ BP-10 10 n.d. 4.7 · 10¹³ 3.0 · 10¹³ BP-100 100 1.1 · 10¹¹ 2.4· 10¹³ 2.0 · 10¹³ BP-1000 1000 1.7 · 10¹⁰ 3.8 · 10¹³ 4.7 · 10¹²BP-10,000 10,000 2.3 · 10⁹  n.m. n.m.

Example 6 (Use Example 4)

[0280] As in Example 5, compound PCH-3N.F.OH was used, but now startingfrom the liquid-crystal mixture C-0. The composition and physicalproperties of the mixtures are shown in the table below (Table 7). TABLE7 Mixture C-0 Composition Compound Concentration/% CC-5-V 20.00 CC-1V-V16.00 PCH-2 12.00 PCH-3 8.00 PCH-3N.F.F 4.00 PCH-301 7.00 CCP-2OCF3 9.00CCP-3OCF3 8.00 CCZU-2-F 6.00 CCZU-3-F 12.00 BCH-32 4.00 CP-33F 4.00

[0281] Physical Properties T(N, I) = 67.5° C. Δn (589 nm) =  0.085 Δε (1kHz) =  7.5 Rotational viscosity = 74 mPa · s V₀ (Frederick No.) =  1.2V

[0282] The specific resistance values of the mixtures are shown in thetable below (Table 8). TABLE 8 Specific resistance of mixture C Mixturec (dopant)/ppm SR (20° C.)/Ωcm C-0 0 5.7 · 10¹¹ CP-100 100 9.2 · 10¹⁰CP-1000 1000 1.8 · 10¹⁰ CP-10,000 10,000 3.0 · 10⁹ 

Example 7 (Use Example 5)

[0283] The liquid-crystal mixture B-0 used in Example 5 was mixed withthe compound

[0284] (abbreviated to BF9-OH). The melting point (transition from thecrystalline phase to the isotropic phase) of this substance is 120° C.10, 100 or 1000 μm of substance BF9-OH were added to three samples ofthe starting mixture B-0, and the specific resistance values of theresultant mixtures BB-10, BB-100 and BB-1000 were compared with those ofthe starting mixture B-0. In addition to the specific resistance valuesin test cells at 20° C., the specific resistance values in test cells at20° C. after heating at 120° C. for 1 hour were also determined. Theresults are shown in Table 9 below. TABLE 9 Specific resistance ofmixture B containing BF9-OH SRZ(120° C., 1 h, Mixture c(BF9-OH)/ppmSRZ(20° C.)/Ωcm 20° C.)/Ωcm B-0 0 3.0 · 10¹³ 1.7 · 10¹³ BB-10 10 2.0 ·10¹³ 1.9 · 10¹³ BB-100 100 2.2 · 10¹³ 2.5 · 10¹³ BB-1000 1000 1.3 · 10¹³1.9 · 10¹²

Example 8: (Use Example 6)

[0285] As in Example 5, PCH-3N.F.OH was used. This was investigated inliquid-crystal mixture D-0, whose composition and properties are shownin Table 10 below. TABLE 10 Composition of mixture D-O CompositionCompound Concentration/% CCP-2F.F.F 5.00 CCP-3F.F.F 9.00 CCP-2OCF3 6.00CCP-3OCF3 6.00 CCP-4OCF3 5.00 CCP-5OCF3 5.00 PCH-3N.F.F 14.00 PCH-5N.F.F4.00 CCZU-2-F 3.00 CCZU-3-F 14.00 CC-5-V 20.00 PCH-302 2.00 CCH-35 4.00BCH-32 3.00

[0286] Physical Properties T(N,I) = 70.0° C. Δn (589 nm) =  0.076 Δε (1kHz) = 10.1

[0287] As in Example 7, the specific resistance values of the mixtureswere measured in test cells. The results are shown in Table 11. TABLE 11Specific resistance of mixture D containing PCH-3N.F.OH in test cellsSRZ (120° C., 1h, Mixture c (dopant)/ppm SRZ (20° C.)/Ωcm 20° C.)/ΩcmD-0 0 1.2 · 10¹³ 1.3 · 10¹³ DP-10 10 1.2 · 10¹³ 1.4 · 10¹³ DP-100 1008.4 · 10¹² 8.0 · 10¹² DP-1000 1000 2.1 · 10¹¹ 4.0 · 10¹¹

Example 9: (Use Example 7)

[0288] As in Example 8, liquid-crystal mixture D was used, but BF9-OHwas now added, as in Example 2. The results of the specific resistancein test cells are shown in Table 12 below. TABLE 12 Specific resistanceof mixture D containing BF9-OH SRZ (120° C., 1h, Mixture c (dopant)/ppmSRZ (20° C.)/Ωcm 20° C.)/Ωcm D-0 0 1.2 · 10¹³ 1.3 · 10¹¹ DB-10 10 1.1 ·10¹³ 1.0 · 10¹³ DB-100 100 3.6 · 10¹² 4.2 · 10¹² DB-1000 1000 4.1 · 10¹¹4.4 · 10¹¹

Example 10: (Use Example 8)

[0289] As in Example 8, mixture D-0 was used, but compound No. 11 ofExample 2c (Substance Example 5)

[0290] CCP-3-CN.OH-F.F.F for short, was now employed. The results forthe specific resistance values in test cells are shown in Table 13below. TABLE 13 Specific resistance of mixture D containingCCU-3-CN.OH—F SRZ (120° C., 1h, Mixture c (dopant)/ppm SRZ (20° C.)/Ωcm20° C.)/Ωcm D-0 0 1.2 · 10¹³ 1.3 · 10¹³ DU-10 10 2.3 · 10¹³ 2.5 · 10¹³DU-100 100 1.9 · 10¹³ 2.1 · 10¹³ DU-1000 1000 1.1 · 10¹³ 1.5 · 10¹³

[0291] The acidity of the compound CCU-3-CN.OH-G is apparently at thelimit of the values to be employed and is just sufficient for someapplications.

Comparative Example 1

[0292] In each case, 1000 ppm of 4-cyano-4′-hydroxybiphenyl,

[0293] OCB for short, of various origin and pretreatment were added toliquid-crystal mixture A-0 from Example 3. The corresponding mixtureswere divided, and in each case one part was filtered as described inExample 4 and in one case repeatedly filtered. The table below (Table14) shows the results for the resistance measurements. TABLE 14Resistance of mixture A containing OCB Origin of No. of Mixture the OCBc (OCB)/ppm filtrations SR/Ω cm A-0 — 0 0 2.7 · 10¹³ AM1-1000 Merck KGaA1 1000 0 3.9 · 10¹⁰ (recrystal- lized AM1-1000 F 1000 1 2.2 · 10¹¹AM2-1000 F Merck KGaA 2 1000 0 1.1 · 10¹¹ (chromato- graphed) AM2-1000 F1000 1 1.9 · 10¹² AA-1000 Aldrich 1000 0 5.3. · 10¹¹ (chromato- graphed)AA-1000 F 1000 1 4.4 · 10¹² AA-1000 1000 0 2.0 · 10¹² AA-1000 F 1000 11.6 · 10¹² AA-1000 FF 1000 2 5.2 · 10¹² AA-1000 FFF 1000 3 6.5 · 10¹²

[0294] As can be seen from Table 14, although the compound OCB issuitable for reducing the resistance of liquid-crystal mixtures, theresults are, however, unreproducible. Firstly, the results depend on theorigin and pre-history (for example the purification method) of the OCB,and secondly, the results change significantly if the mixtures aresubjected to standard operations, for example filtration.

Comparative Example 2

[0295] Various amounts of2,6-bis-tert-butyl-4-(4-n-propyltrans-cyclohexyl)phenol

[0296] TBCP for short, were added to liquid-crystal mixture E-0.

[0297] Mixture E-0 has the composition shown below (Table 15) and hasthe properties described therein. TABLE 15 Mixture E-O CompositionCompound Concentration/% PCH-5F 10.00 PCH-6F 8.00 PCH-7F 6.00 CCP-20CF38.00 CCP30CF3 12.00 CCP-40CF3 7.00 CCP-50CF3 11.00 BCH-3F.F 12.00BCH.5F.F 10.00 ECCP-30CF3 5.00 ECCP-50CF3 5.00 CBC-33F 2.00 CBC-53F 2.00CBF-55F 2.00

[0298] Physical Properties

[0299] T(N, 5)=92° C.

[0300] Δn (589 nm)=0.097

[0301] Δε (1 kHz)=5.2

[0302] V₁₀=2.0 V

[0303] For the mixtures having the various concentrations, the voltageholding ratio was determined as in Example 4. The alignment layer herewas PI-1 (AL-1051 from Japan Synthetic Rubber). The results are shown inthe table below (Table 16). TABLE 16 Voltage holding ratio for mixture EContaining TBCP Mixture C (TBCP)/ppm HR (20° C.)/% E-0 0 99.7 ET-10001000 97.6 ET-2500 2500 97.4 ET-5000 5000 96.8

[0304] As can be seen from the results in Table 11, the compound TBCP isnot suitable for reducing the specific resistance or voltage holdingratio of liquid-crystal mixtures. This is probably due to its low pK_(a)value, i.e. its low acidity.

Comparative Example 3

[0305] Various concentrations of the compound 4-cyano-3-fluorophenol,

[0306] CFP for short, were dissolved in liquid-crystal mixture A fromExample 3, giving the specific resistance values shown in the tablebelow TABLE 17 Specific resistance of mixture A containing CFP Mixture c(CFP)/ppm SR/Ω cm A-0 0 1.2 · 10¹³ ACF-1 1 5.3 · 10¹² ACF-10 10 2.7 ·10¹² ACF-100 100 8.3 · 10¹¹ ACF-1000 1000 1.0 · 10¹¹ ACF-5000 5000 1.3 ·10¹⁰ ACF-10,000 10,000 4.8 · 10⁹

[0307] As can be seen from the results in Table 12, the compound CFP,which has a relatively high pK_(a) value, can be used to reduce thespecific resistance of liquid-crystal mixtures. The voltage holdingratio was then determined in liquid-crystal cells containing PI-1. Theresults are shown in Table 18. TABLE 18 Voltage holding ratio of mixtureA containing CFP Mixture c (CFP)/% HR (20° C.)/% HR (100° C.)/% D-0 099.7 98.6 A-0 0 98.8 87.9 ACF-1 1 99.4 89.1 ACF-10 10 99.4 89.2 ACF-100100 98.4 86.8

[0308] It can be seen that, particularly at low CFP concentrations,there is at best a small effect, if any at all. Furthermore, sampleswere investigated after various storage times and after various times inopened containers under nitrogen gas and after various times in thevacuum cell-filling unit. It was found that the resistance of themixtures containing CFP increases significantly in particular on storagein open containers under nitrogen and very particularly in the vacuumfilling unit, and the voltage holding ratio likewise increases. Thisbehaviour can probably be attributed to the high vapour pressure of thecompound CFP. This makes it unsuitable for practical use, although, asshown in Table 12, it does in some cases have the desired effect on theresistance of liquid-crystal mixtures.

Comparative Example 4

[0309] Analogously to Comparative Example 3, the compound4-ethoxy-3-cyano-2-fluorophenol,

[0310] ECFP for short, was investigated in the liquid-crystal mixtureA-0. Very similar results were obtained. Like CFP, ECFP also results ina significant reduction in the specific resistance which is clearlydependent on the concentration employed. The effect is somewhat greaterin the case of ECFP than in the case of CFP. However, the problem of theexcessive vapour pressure also occurs for ECFP, albeit to a somewhatlesser extent than for CFP. Like CFP, ECFP is therefore not particularlysuitable for practical applications.

Comparative Example 5

[0311] The compound 2-cyano-4-(4-n-propyl-trans-cyclohexyl)phenol,

[0312] PCH-3N.OH,

[0313] prepared analogously to the compound PCH-3N.F.OH in Example 1,has a melting point of 145.4° C. As in Example 3, the specificresistance of PCH-3N.OH was determined in mixture A-0. The results areshown in Table 19. TABLE 19 Specific resistance of mixture A containingPCH-3N.OH Mixture c (PCH-3N.OH)/ppm SR/Ωcm A-0 0 1.4 · 10¹³ AN-1000 10001.0 · 10¹²

[0314] Compared to Example 3(Table 2, mixture A-1000), it can be seenthat 1000 ppm of PCH-3N.F.OH reduce the specific resistance of mixtureA-0 only from 1.9·10¹³ Ωcm to 1.0·10¹² Ωcm, whereas the compound ofExample 1, PCH-3N.OH, in the same concentration reduces the specificresistance to 1.0·10¹¹ Ωcm. Owing to the lower acidity compared withPCH-3N.F.OH, PCH-3N.OH is therefore not as suitable for adjusting thespecific resistance of liquid-crystal mixtures, such as mixture A-0.

Comparative Example 6

[0315] As in Example 8, mixture D-0 was used. Now, however, the compound

[0316] CCP-3OH-F.F.F for short, was employed. The results for thespecific resistance values in the test cells are shown in Table 20below. TABLE 20 Resistance of mixture D containing CCP-3OH-F.F.F c(dopant)/ SRZ (20° C.)/ SRZ (120° C., 1 h, 20° C.)/ Mixture ppm Ωcm ΩcmD-0 0 1.2 · 10¹³ 1.3 · 10¹³ DC-10 10 9.8 · 10¹² 1.1 · 10¹³ DC-100 1001.4 · 10¹³ 1.2 · 10¹³ DC-1000 1000 8.7 · 10¹² 1.1 · 10¹³

[0317] As can be seen from the results, the compound CCP-30H-F.F.F isnot suitable for adjusting the resistance. This seems to be attributableto its inadequate pK_(a) value.

Comparative Example 7

[0318] As in Example 8, mixture D-0 was used. Now, however, the compoundadded was

[0319] LUU-3-OH for short, was employed. The results are shown in Table21 below. TABLE 21 Resistance of mixture D containing LUU-3-OH c(dopant)/ SRZ (20° C.)/ SRZ (120° C., 1 h, 20° C.)/ Mixture ppm Ωcm ΩcmD-0 0 1.2 · 10¹³ 1.3 · 10¹³ DL-10 10 1.9 · 10¹³ 2.0 · 10¹³ DL-100 1002.2 · 10¹³ 2.0 · 10¹³ DL-1000 1000 1.7 · 10¹³ 1.6 · 10¹³

[0320] Like dopant CCP-30H-F.F.F from Comparative Example 6, LUU-3-OH isalso unsuitable for adjusting the resistance.

[0321] The entire disclosure of all applications, patents andpublications, cited above, and of corresponding German application Nos.198 04 300.7 filed Feb. 4, 1998, 198 05 912.4 filed Feb. 13, 1998 and198 51 805.6 filed Nov. 11, 1998 are hereby incorporated by reference.

[0322] The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

[0323] From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. Liquid-crystal mixture having a certain specific resistance,characterized in that it comprises an acidic compound in a concentrationof from 10 ppm to less than 10%.
 2. Liquid-crystal mixture according toclaim 1, characterized in that the acidic compound is a phenol. 3.Liquid-crystal mixture according to claim 1, characterized in that theacidic compound is a phenol of the formula I

in which A¹ and A² are each, independently of one another and in thecase where A¹ occurs a number of times, these too are each,independently of one another: a) 1,4-cyclohexylene ortrans-1,4-cyclohexenylene, in which, in addition, one or morenon-adjacent CH₂ groups may be replaced by O and/or S, b) 1,4-phenylene,in which, in addition, one or two CH groups may be replaced by N, c)1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl,decahydronaphthalene-3,6-diyl or 1,2,3,4-tetrahydronaphthalene-2,6-diyl, in which (a) and (b) are, independently, unsubstituted, monosubstitutedor disubstituted by F atoms, Z¹ and Z² are each, independently of oneanother and in the case where Z occurs a number of times, these too areeach independently of one another: —CO—O—, —O—CO—, —CO—CH₂—, —CH₂—CO—,—CH₂O—, —OCH₂—, —CH₂CH₂—, —CH═CH—, —C≡C— or a single bond, or one of thegroups Z and Z is —(CH₂)₄—, —(CH₂)₃CO—, —(CH₂)₂—O—CO—, —(CH₂)₂—(CO—O)—,CH═CH—CH₂CH₂—, —CH₂—CH₂— CH═CH— or —CH₂—CH═CH—CH₂—, R is H, alkyl oralkenyl having 1 or 2 to 15 carbon atoms respectively, which areunsubstituted, monosubstituted by CN or CF₃ or monosubstituted orpolysubstituted by halogen, in particular F or Cl, where, in addition,one or more CH₂ groups in these radicals may be replaced, independentlyof one another, by —O—, —S—,

 —CO—, —CO—O—, —O—CO— or —C—CO—C—in such a way that no two O atoms arebonded directly to one another, or CN, F, Cl or COOR′, or OH, R′ is H orR, where CN, F, OH and COOR′ are excluded, n is 0, 1 or 2, m is 0 or 1,o is 2 or 3, n+m+o is 2, 3 or 4, X and Y are each, independently of oneanother and in the case where X and/or Y occur a number of times, thesetoo are each independently of one another, F, Cl, COOR′, NO₂ or CN, p₁,p₂, q₁ and q₂ are each 0, 1, 2, 3 or 4, where o is 1 and q₂ is 0 p₂ is 3or 4 where o is 1 and q₂ is 1, 2 or 3 p₂+q₂ is 1, 2, 3 or 4 where o is 2or 3 p₁+p₂+ q₁+q₂ is 1, 2, 3, 4, 5, 6, 7, or 8, where q₁+q₂ is 1 or 2p₁+p₂ is 1, 2, 3 or 4 where q₁+q₂ is 0 p₁+p₂ is 3, 4, 5, 6, 7 or 8
 4. Amethod of using a liquid-crystal mixture according to claim 1 whichcomprises incorporating said liquid-crystal mixture according to claim 1in a liquid-crystal display.
 5. A liquid-crystal display containing aliquid-crystal mixture according to claim
 1. 6. A liquid-crystal displayaccording to claim 5, characterized in that it is an STN, AMD, TN or IPSdisplay.
 7. A method of adjusting the specific resistance ofliquid-crystal mixtures, characterized in that acidic compounds areadded to the liquid-crystal mixture.
 8. A method according to claim 7,characterized in that the acidic compounds are mesogenic compounds oflow vapor pressure and high acidity (pk_(a)).
 9. A method of adjustingthe specific resistance of liquid-crystal mixtures, characterized inthat acidic compounds of the formula I as described in claim 3 are addedto the liquid crystal mixture.
 10. Compounds of the formula I given inclaim 3, with the proviso that compounds of the formulae

are excluded.
 11. Process for the preparation of compounds of theformula I according to claim 10, characterized in that the phenolic OHgroup is introduced by nucleophilic substitution of substituents suchas, for example, halogen.
 12. A liquid-crystal mixture according toclaim 3 characterized in that the acid compound is a phenol of theformula Ia′ or Ib′

wherein A¹, A², Z¹, Z², R¹, n, m, o, x p₁, p₂, q₁, q₂ are as defined inclaim 3, p+q is 1, 2, 3, 4, 5, 6, 7 or 8, and where q=0 p is 3, 4, 5, 6,7 or 8 and where q=1 p is 1, 2, 3, 4, 5, 6 or 7, and each ab is 0 or 1subject to the proviso that at least one ab is
 1. 13. A liquid crystalmixture as in claim 1 having a value for specific resistance within therange of 10⁹ Ωcm to 10¹³ Ωcm.